Geochronology U/Pb and Sm/Nd of the Edough metamorphic formations (NE Algeria)

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Miocene incorporation of peridotites into the lower crust during opening of the Algerian basin: insight from U-Pb LA-ICP-MS analyses

Sizeable pieces of deep mantle material croup-out worldwide within orogenic belts involving major continental collisions or within regions with rift-thinned continental margins. From their original mantle position these peridotites have been transferred into the crust by tectonic processes involving subduction, collision or extension and, although minor components of most metamorphic belts, the understanding of how and when they were emplaced within the continental crust and their subsequent exhumation is paramount. A U-Pb age of 17.84±0.12 Ma (2σ) was obtained from monazites separated from a diatexite migmatite collected in close proximity to a small peridotite massif incorporated into the lower crustal sequence of the Edough massif (eastern Algeria), a southern segment of the peri-Mediterranean Alpine Belt. Monazites, extracted from a neighbouring deformed leucogranite, yield a similar age of 17.4±1.3 Ma (2σ) whereas coexisting zircon with magmatic characteristics yield an age of 308±7 Ma (2σ) interpreted as dating the magmaticcrystallisation of the leucogranite during the hercynian orogeny. These results emphasize the polycyclic evolution of basement rocks preserved in the crystalline units of the Western Mediterranean and indicate that part of their metamorphic features was inherited from older events. Taken together with published Ar-Ar datings, the late Burdigalian age of monazites, indicates a fast cooling rate of c. 300°C/Ma and is regarded as closely approximating the emplacement of the peridotites into the hercynian basement. This age issignificantly younger than those recorded for orogenic peridotites from the Betic-Rif orocline and for lithospheric extension forming the Alboran sea. It is also younger than rifting and back-arc extension opening the Liguro-Provençal basin. The late Burdigalian age is interpreted as dating the incipient rifting event that opened the Algerian basin. At the scale of the Western Mediterranean, opening of the Algerian basin provides a link between the late Oligocene-early Miocene extension in the western part (Alboran basin, Valencia trough and Liguro-Provençal basin) and the upper Miocene extension in the eastern part (Thyrrenian basin) of the western Mediterranean

Monazite “in situ” 207Pb/206Pb geochronology using a small geometry high-resolution ion probe. Application to Archaean and Proterozoic rocks

International audienceThis paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion probe to provide reliable in situ 207Pb/206Pb ages on monazite populations of Archaean and Proterozoic age. The reliability of the SIMS technique has been assessed on two samples previously dated by the conventional ID-TIMS method at 2661±1 Ma for monazites extracted from a pelitic schist from the Jimperding Metamorphic Belt (Yilgarn Craton, Western Australia) and 1083±3 Ma for monazites from a high-grade paragneiss from the Northampton Metamorphic Complex (Pinjarra Orogen, Western Australia). SIMS results provide 207Pb/206Pb weighted mean ages of 2659±3 Ma (n=28) and 1086±6 Ma (n=21) in close agreement with ID-TIMS reference values for the main monazite growth event. Monazites from the Northampton Complex document a complex history. The spatial resolution of about 30 μm and the precision achieved successfully identify within-grain heterogeneities and indicate that monazite growth and recrystallisation occurred during several events

Sampling an active continental paleo-margin: a LA-ICP-MS U-Pb zircon study from the Adrar des Iforas (Mali)

The Tuareg Shield results from the amalgamation, during the Pan-African orogeny, of more than twenty terranes comprising Neoproterozoic juvenile oceanic terranes and reworked or well-preserved Archean and Paleoproterozoic crustal blocks. All these amalgamated terranes are limited by trans-continental N–S shear-zones. Their main relative movements resulted from a Northern transpressional tectonic escape due to their squeezing between the West African craton (WAC) to the west and the East Saharan craton to the East. This configuration hampers tectonic correlations through the shield, but gives rise to major issues including the way along which amalgamation of the terranes occurred, their respective tectonometamorphic evolution and their origin. In order tohelp unravel the tectonometamorphic evolution of the westernmost part of the shield, we performed LA-ICPMS UPb zircon geochronology on samples from key lithologies collected through the Adrar des Iforas area (Mali). Samples were selected in order to represent a Neoproterozoic Andeantype active margin developed during subduction of the WAC beneath the Iforas terrane of the Tuareg Shield. In the Central Iforas Belt, arc-related calc-alkaline magmatism occurred as early as 716±6 Ma and was coeval with the 710-730 Ma old Tilemsi intraoceanic arc. Subduction-related magmatism persisted until 634±8 Ma and possibly down to 623±6 Ma, theage of the Tafeliant metadacite. Calc-alkaline batholiths were emplaced into a pre-Panafrican gneissic basement containingPaleoproterozoic units (such as the Tafeliant syenitic gneiss dated at 1966±9 Ma) and Archean components (inheritance in granitoids at c. 2.4-2.5 Ga and up to 2.8 Ga). Emplacement of syn-kinematic granitic plutons, associated to collision with the WAC, occurred at 604±5 Ma, whereas late-kinematic emplacement of the central batholith is dated at 599±3 Ma. East of the Adrar fault, the basement of the Eastern Iforas Belt contains Paleoproterozoic granitoids (Tin Essako orthogneiss dated at 2020±5 Ma), which were intruded by pre-kinematic granitoids such as the 630±6 Ma Tamaradant metadiorite. This suggests a possible synchronism of ante-collisional events oneither sides of the Iforas Granulitic Unit

Origin and signficance of basic and ultrabasic outcrops from Northeastern Algeria (Edough massif)

International audienceThe Maghrebian, Betics and Apenninic chains outcrop around the mediterranean basin and constitute dismembered fragments of the Alpine orogen. Conflicting geodynamic models have been proposed in order to explain the different paleogeographic settings from which these fragments derived.Crust-mantle interactions following subduction of Jurassic oceanic crust and collision-related tectonic events of Eocene age in relation to the northward motion of Africa have been demonstrated by numerous works. The incorporation of mafic/ultramafic rocks into the basement is evidenced in various peri-Mediterranean areas, in particular at c. 22 Ma. This work is focused on the basic and ultrabasic rocks from the Easternmost internal part of the Maghrebides. An extensive petrological and geochemical study has been performed on three distinct outcrops, i.e. Bou Maiza gabbros, amphibolitesfrom La voile Noire and Sidi Mohamed peridotites. Peridotites display a primitive character (Mg number >85), but slightly enriched trace elements patterns (1 to 10 times CHUR) characterized by negative Nb anomalies and flat to slightly LREE-depleted patterns. Associated isotopic constraints suggest a possible continental contamination of the peridotites by the surrounding gneisses. These ultrabasic rocks are interpreted as parts of the lithospheric mantle incorporated into the continental crust during a late Burdigalian extensional event that opened the Algerian basin. The Bou Maiza gabbros and La Voile Noire amphibolites show complementary trace elements spectra suggesting derivation from a common MORB source reservoir, but without filiation with the Sidi Mohamed peridotites. Such affinities suggest they represent a fragment of the Neothetys lithosphere obducted onto the North African margin during Miocene times

Orogenic development of the Adrar des Iforas (Tuareg Shield, NE Mali): New geochemical and geochronological data and geodynamic implications

International audienceLaser-ablation U–Th–Pb analyses of zircon and allanite from magmatic and metamorphic rocks of the Adrar des Iforas in Northern Mali allow re-examining the relationships between the different crustal units constituting the western part of the Tuareg Shield, as well as the timing of magmatic and metamorphic events in the West Gondwana Orogen. Granulite-facies metamorphism in the Iforas Granulitic Unit (IGU) and at In Bezzeg occurred at 1986 ± 7 Ma and 1988 ± 5 Ma respectively. This age is slightly younger, but consistent with that of the HT granulite facies event characterizing the In Ouzzal granulitic unit (IOGU), thereby substantiating the view that these units once formed a single granulitic belt of c. 800 km long. High-grade metamorphic basement units of the Kidal terrane surrounding the IGU contain Paleoproterozoic magmatic rocks crystallized between 1982 ± 8 Ma and 1966 ± 9 Ma. Inherited components in these rocks (2.1 Ga and 2.3–2.5 Ga) have ages similar to that of detrital zircons at In Bezzeg and to that of basement rocks from the IGU. This is taken as evidence that the Kidal terrane and the IGU formed a single crustal block at least until 1.9 Ga. East of the Adrar fault, the Tin Essako orthogneiss is dated at 2020 ± 5 Ma, but escaped granulite facies metamorphism. During the Neoproterozoic, the Kidal terrane underwent a long-lived continental margin magmatism. To the west, this terrane is bounded by the Tilemsi intra-oceanic island arc, for which a gneissic sub-alkali granite was dated at 716 ± 6 Ma. A synkinematic diorite extends the magmatic activity of the arc down to 643 ± 4 Ma, and, along with literature data, indicates that the Tilemsi arc has a life span of about 90 Ma. Backward docking to the western margin of the Kidal terrane is documented by migmatites dated at 628 ± 6 Ma. Subduction related processes and the development of the Kidal active margin was responsible for the development of a back-arc basin in the Tafeliant area, with deposition of sediments intruded by basaltic and dacitic lavas, one of which was dated at 623 ± 6 Ma. East of the IGU, in the Tamaradant domain, metagabbros and meta-anorthosites emplaced within greywackes have geochemical characteristics typical of subduction-related environments (enrichment in LILE and HFSE depletion). A metadiorite intruding the Tamaradant sediments gave an age of 630 ± 6 Ma, which is tentatively taken as evidence for a Pan-African age for the subduction processes that took place east of the IGU. Collision of the Kidal terrane with the eastern margin of the West African Craton is best dated by a syncollisional tonalite, which provides an age of 604 ± 5 Ma. Late kinematic processes shortly followed at 599 ± 4 Ma as exemplified by the emplacement of a monzogranite belonging to the complex Central Batholith. The geochronological and geochemical evidences provided by this study allow proposing that the Kidal terrane, the IGU and Tamaradant domain of the Adrar des Iforas once belonged to a single terrane, which probably extended northward to include the IOGU, and which was later dissected by major lithospheric scale faults during the late Pan-African orogenic phases

Reworking of intra-oceanic rocks in a deep sea basin: example from the Bou-Maiza complex (Edough massif, eastern Algeria)

International audienceMetagabbros and amphibolites exposed in the Bou-Maïza area of the Edough massif (northeast Algeria) are described in detail. Field and petro-structural observations point to the syn-sedimentary emplacement of gabbros as clasts, blocks and lenses of polymictic gabbroic breccias. Associated amphibolites display fine-scale parallel sedimentary bedding and represent mafic epiclastites, litharenites and mafic greywackes. The mafic beds and lenses are intercalated with aluminous pelitic schists of continental origin, quartzite and marble. It is concluded that all mafic rocks from this locality derive from the erosion of an oceanic plutono-volcanic complex of MORB affinity that was reworked in a block matrix mélange and emplaced as turbidites and debris flows during the Mesozoic. We propose a convergent plate margin setting for these formations connected with the subducted Calabrian branch of the Tethyan slab

Geochemical study (major, trace elements and Pb-Sr-Nd isotopes) of mantle material obducted onto the North African margin (Edough Massif, North Eastern Algeria): Tethys fragments or lost remnants of the Liguro-Provencal basin?

International audienceThe Maghrebides, Betics and some parts of the Calabrian, NE Sicilian and Tuscan allochtons constitute dismembered fragments of the Alpine belt that resulted from the Cenozoic collision between Africa and Eurasia and the opening of the Western Mediterranean basin. Mineral and whole-rock geochemical analyses have been performed on three distinct outcrops of mantle material from the Edough Massif of NE Algeria, namely the Bou Maiza (BM) gabbros, the La Voile Noire (LVN) amphibolites and the Sidi Mohamed (SM) peridotites. In all samples, Sr isotopes are largely affected by seawater alteration (87Sr/86Sract. > 0.70384 and up to 0.70888) and cannot be used to evaluate the nature of the source reservoirs. SM peridotites display variable depleted mantle Nd isotopic signatures (εNdact. from + 7.0 to + 12.2) and geochemical features suggesting no significant chemical depletion as a result of partial melting and melt extraction (Mg# + 7.9) and are likely cogenetic, but without filiation with the SM peridotites. Pb isotopes indicate a contribution of sediments in the source reservoir, which is attributed to contamination solely by hydrous fluids released from a sedimentary component. This observation, together with a LILE-enrichment, suggests a back-arc basin environment. These results indicate that BM and LVN units were obducted onto the North African margin and subsequently fragmented, probably during doming and exhumation of the lower continental crust of the Edough massif. Doming resulted in opposite movements of the overlying oceanic units, southward for the BM units and northward for LVN amphibolites

Discovery of metamorphic diamonds in garnets from the Edough massif (Northeastern Algeria): LA-ICP-MS U-Pb constraints

International audienceInclusions in a garnet megacryst (≥ 5 cm) from the Edough Massif (NE Algeria) have been studied and we report for the first time the discovery of diamond inclusions (5-30 micrometer in size), identified by Raman spectroscopy and the characteristic sharp band at 1332 cm-1 for crystalline diamond. The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite-ultramyloniteband. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. This 1 km thick slab of amphibolites and pyrigarnites, derived from layered gabbros of MORB affinity, rest above the granite-gneiss units from the core complex. Garnet-forming reactions and dehydration melting are the oldest metamorphic stages recognized in the Kef Lakhal mafic complex (≥ 800 ◦C, ≥ 17 Kbar). The diamond-bearing garnets display a dense rutile acicular network interpreted as exsolutions. Zircons inclusions have been also observed in this garnet megacryst. Diamond crystals, up to 50 micrometer across, were identified first by optical microscopy and later by Raman spectroscopy. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300 micrometer in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724-778 ◦C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥ 3.6 GPa that were reached during subduction of the UHP-rock precursors. In situ U-Pb dating obtained on rutile inclusions yield a 32.4+/ 3.3 Ma interpreted as the age of the UHP metamorphic event. LA-ICP-MS U-Pb ages on zircon provide a 20.9 +/- 2.2 Ma age attributed to the thrusting onto the North-African margin. We suggest that the mafic protolith of the analyzed garnet megacryst originates from the subducted retreating Tethyan slab, which broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by building up of the Edough dome and opening of the Algerian basin

Metamorphic diamonds in a garnet megacryst from the Edough Massif (northeastern Algeria). Recognition and geodynamic consequences

International audienceWe report for the first time the discovery of diamond inclusions (5–30 μm in size), identified by Raman spectroscopy and the characteristic sharp band at 1332 cm− 1 for crystalline diamond, in a garnet megacryst (≥ 5 cm) from the Edough Massif (NE Algeria). The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite–ultramylonite band. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. The host garnet is almandine-dominant and is rich in exsolution of acicular rutile needles. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300 μm in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724–778 °C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥ 3.6 GPa that were reached during subduction of the UHP-rock precursors. We suggest that the mafic protolith originates from the subducted retreating Calabrian branch of the Tethyan slab, that broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by formation of the Edough dome and opening of the Algerian basin